1. Field of the Invention
This invention relates generally to structures and devices produced by techniques of nanotechnology
More specifically, the invention relates to such structures and devices incorporating at least one element, essentially in one-dimensional form, and that is of nanometer dimensions in its width or diameter, and that is produced by the so-called Vapour-Liquid-Solid (VLS) mechanism. For the purposes of this specification, such element will be termed a “nanowhisker”.
2. Brief Description of the Prior Art
Nanotechnology covers various fields, including that of nanoengineering, which may be regarded as the practice of engineering on the nanoscale. This may result in structures ranging in size from small devices of atomic dimensions, to much larger scale structures for example on the microscopic scale. Commonly, such structures include nanostructures. Typically, nanostructures are devices having at least two dimensions less than about 1 μm (i.e., nanometer dimensions). Ordinarily, layered structures or stock materials having one or more layers with a thickness less than 1 μm are not considered to be nanostructures. Thus the term nanostructures includes free-standing or isolated structures having two dimensions less than about 1 μm that have functions and utilities that are different from larger structures and are typically manufactured by methods that are different from conventional procedures for preparing somewhat larger, i.e., microscale, structures. Thus, although the exact boundaries of the -class of nanostructures are not defined by a particular numerical size limit, the term has come to signify such a class that is readily recognized by those skilled in the art. In many cases, an upper limit of the size of the at least two dimensions that characterize nanostructures is about 500 nm. In some technical contexts the term “nanostructure” is construed to cover structures having at least two dimensions of about 100 nm or less. In a given context the skilled practitioner will recognize the range of sizes intended. In this application the term “nanostructure” or “nanowhisker” is intended to refer to an elongated structure having at least two transverse dimensions less than about 1 μm, as indictated above.
Nanostructures include one-dimensional nanoelements, essentially in one-dimensional form, and that are of nanometer dimensions in their width or diameter, and that are commonly known as a nanowhisker, nanorods, nanowires, nanotubes, etc.
As regards nanowhiskers, the basic process of whisker formation on substrates, by the so-called VLS (vapour-liquid-solid) mechanism, is well known. A particle of a catalytic material, usually gold, for example, on a substrate is heated in the presence of certain gases to form a melt. A pillar forms under the melt, and the melt rises up on top of the pillar. The result is a whisker of a desired material with the solidified particle melt positioned on top. (See E. I Givargizov, Current Topics in Materials Science, Vol. 1, pages 79-145, North Holland Publishing Company, 1978.) The dimensions of such whiskers were in the micrometer range.
Although the growth of nanowhiskers catalyzed by the presence of a catalytic particle at the tip of the growing whisker has conventionally been referred to as the VLS (Vapour-Liquid-Solid process), it has come to be recognized that the catalytic particle may not have to be in the liquid state to function as an effective catalyst for whisker growth. At least some evidence suggests that material for forming the whisker can reach the particle-whisker interface and contribute to the growing whisker even if the catalytic particle is at a temperature below its melting point and presumably in the solid state. Under such conditions, the growth material, e.g., atoms that are added to the tip of the whisker as it grows, may be able to diffuse through a the body of a solid catalytic particle or may even diffuse along the surface of the solid catalytic particle to the growing tip of the whisker at the growing temperature. Evidently, the overall effect is the same, i.e., elongation of the whisker catalyzed by the catalytic particle, whatever the exact mechanism may be under particular circumstances of temperature, catalytic particle composition, intended composition of the whisker, or other conditions relevant to whisker growth. For purposes of this application, the term “VLS process”, or VLS mechanism, or equivalent terminology, is intended to include all such catalyzed procedures wherein nanowhisker growth is catalyzed by a particle, liquid or solid, in contact with the growing tip of the nanowhisker.
International Application Publication No. WO 01/84238 discloses in FIGS. 15 and 16 a method of forming nanowhiskers wherein nanometer sized particles from an aerosol are deposited on a substrate and these particles are used as seeds to create filaments or nanowhiskers.
For the purposes of this specification the term nanowhiskers is intended to mean one-dimensional nanoelements with a width or diameter (or, generally, a cross-dimension) of nanometer size, the elements having been formed by the so-called VLS mechanism. Nanowhiskers are also referred to in the art as “nanowires” or, in context, simply as “wires”, and such terminology, as used in this application, is equivalent to the term “nanowhiskers”
Positioning whiskers at selected positions on a substrate is desirable for an integration of whisker technology with current semiconductor component technology.
Several experimental studies on the growth of nanowhiskers have been made, the most important reported by Hiruma et al. They grew III-V nano-whiskers on III-V V substrates in a metal organic chemical vapor deposition (MOCVD) growth system. (See K. Hiruma, et al., J. Appl. Phys. 74, page 3162 (1993); K. Hiruma, et al., J. Appl. Phys. 77, page 447 (1995); K. Hiruma, et al., IEICE Trans. Electron. E77C, page 1420 (1994); K. Hiruma, et al., J. Crystal Growth 163, pages 226-231 (1996)) Their approach relied on annealing a thin Au film to form the seed particles. In this way, they achieved a homogeneous whisker width distribution, the mean size of which could be controlled by the thickness of the Au layer and the way this layer transforms to nanoparticles. With this technique, it is difficult to control the size and surface coverage separately.
Sato et al., Appl. Phys. Lett. 66(2), 9 Jan. 1995, disclose an attempt to grow four nanowhiskers, the position or site of each nanowhisker being predetermined by use of four respective windows in an SiO2 mask. The mask was then removed and growth was initiated. The results were, in general, unsatisfactory for producing a structure with nanowhiskers sufficiently precisely positioned for electronics/photonics components, one reason being that the nanowhisker growth site could not be accurately localised, on account of the use of a thin film for forming the seed particle. Further the structure consisting of four nanowhiskers was not extended to include a large number of nanowhiskers such as is necessary for many applications.
U.S. Pat. No. 5,332,910 discloses a micro-cavity laser comprising a plurality of semiconductor rods arranged at a distance equal to an integer sub-multiple of light emitted from the semiconductor rod. In one embodiment, a focussed ion beam was used to create seeds in a substrate from which nanowhiskers were grown. In another embodiment, nanowhiskers were grown by MOCVD in etched holes in an SiO2 mask.
A component that is of considerable interest is a photonic bandgap array. This comprises an array of a large number of elements (that may be projections or holes) in a substrate and that provide a different refractive index from the surrounding medium. Selective area MOVPE growth (SA-MOVPE) has been used to develop two-dimensional photonic bandgap structures, but these appear of limited applicability. (See Akabori et al., Physica E13, pages 446-450, March 2002).
Work has also been carried out for forming photonic band gap structures from carbon nanotubes, where single-seeded arrays have been synthesized using methods such as self-assembly nanosphere lithography and optical lithography. (See K. Kempa et al., Letters, Vol. 3, No. 1, pages 13-18, 19 Nov. 2002). However tubes are not ideal structures for forming areas of different refractive index.
In co-pending U.S. patent application Ser. No. 10/613,071, filed Jul. 7, 2003, the contents of which are incorporated herein by reference and co-pending International Application PCT/GB2003/002929, a method of producing arrays of nanowhiskers is described suitable for use as photonic bandgap structures.